Journal of Experimental Botany, Vol. 54, No. 385, pp. 1221-1229,
April 1, 2003
© 2003 Oxford University Press
Root and stem hydraulic conductivity as determinants of growth potential in grafted trees of apple (Malus pumila Mill.)
Received 23 August 2002; Accepted 14 January 2003
Crop Science Department, Horticulture Research International, East Malling, West Malling, Kent ME19 6BJ, UK
1 To whom correspondence should be addressed. Fax: +44 (0)1732 849067. e-mail: chris.atkinson{at}hri.ac.uk
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Abstract |
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The anatomy of the graft tissue between a rootstock and its shoot (scion) can provide a mechanistic explanation of the way dwarfing Malus rootstocks reduce shoot growth. Considerable xylem tissue disorganization may result in graft tissue having a low hydraulic conductivity (kh), relative to the scion stem. The graft may influence the movement of substances in the xylem such as ions, water and plant-growth-regulating hormones. Measurements were made on 3-year-old apple trees with a low-pressure flow system to determine kh of root and scion stem sections incorporating the graft tissue. A range of rootstocks was examined, with different abilities of dwarfing; both ungrafted and grafted with the same scion shoot cultivar. The results showed that the hydraulic conductivity (khroot) of roots from dwarfing rootstocks was lower compared with semi-vigorous rootstocks, at least for the size class of root measured (1.5 mm diameter). Scion hydraulic conductivity (khs) was linked to leaf area and also to the rootstock on to which it was grafted, i.e. hydraulic conductivity was greater for the scion stem on the semi-vigorous rootstock. Expressing conductivities relative to xylem cross-sectional areas (ks) did not remove these differences suggesting that there were anatomical changes induced by the rootstock. The calculated hydraulic conductivity of the graft tissue was found to be lower for grafted trees on dwarfing rootstocks compared to invigorating rootstocks. These observations are discussed in relation to the mechanism(s) by which rootstock influences shoot growth in grafted trees.
Key words: Apple, dwarfing, graft tissue, growth control, hydraulic conductivity, Malus, rootstock.
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Introduction |
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Changes in graft anatomy are often evident when stem tissue (scion, the part of the plant used for grafting on a stock) of perennial woody species is grafted onto a rootstock (stem tissue and associated roots from another plant) that restricts its vegetative growth (Warne and Raby, 1938; Simons, 1986; Soumelidou et al., 1994a). The mechanism(s) by which Malus rootstocks influence scion vegetative growth and development, and vice versa, are not fully understood (Beakbane, 1956; Tubbs, 1973a, b; Jones, 1986).
Some authors have suggested that the graft tissue influences vegetative shoot growth by restricting water flow from the root to the shoot or by removing substances, particularly minerals and plant growth regulators (i.e. cytokinins), from the transpiration stream (Knight, 1926; Jones, 1974, 1984). This may not be the case with all composite plants, for example, some Vitis combinations, when self-grafted, have lower hydraulic conductivities compared with on their own roots (Bavaresco and Lovisolo, 2000). A restriction of water flow is entirely consistent with the anatomical changes associated with graft tissues and the different degrees of shoot dwarfism shown in grafted plants (Mosse, 1962; Simons, 1986; Soumelidou et al., 1994a). These anatomical changes may be due to limitations in polar auxin (IAA) transport across the graft and its accumulation at the graft (Soumelidou et al., 1994b; Simons, 1986); IAA is a key leaf-derived regulator of xylem cell differentiation and division within the cambial zone and an initiator of vascular redifferentiation across the graft union (Parkinson and Yeoman, 1982; Hess and Sachs, 1972; Aloni, 1987; Savidge, 1988). A reduced flow of IAA to roots could provide an explanation of the observed changes in the phloem-to-xylem ratio in apple rootstocks (Beakbane, 1956). A similar argument has been made for the differentiation of water-transporting tracheids in graft tissue of Picea sitchensis (Weatherhead and Barnett, 1986). There is considerable evidence (Beakbane and Thompson, 1947; Simons, 1986), with perennial fruit trees, that the rootstocks that dwarf shoots have a lower xylem-to-phloem ratio compared with rootstocks that promote shoot growth (Beakbane and Thompson, 1947). The consistency of this ratio has enabled it to be used successfully for seedling selection in apple rootstock breeding programmes (Beakbane and Thompson, 1947; Miller et al., 1961).
The objectives of this research were (i) to determine the relationship between scion stem hydraulic conductivity (kh) and the ability of rootstocks to restrict shoot growth, and (ii) to determine whether the kh of graft tissues is a means by which shoot growth is influenced in grafted trees of Malus.
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Materials and methods |
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Plant material and experimental design
Clonally produced 1-year-old material (Malus pumila Mill.) of a dwarfing (M.27) and a semi-vigorous rootstock type (MM.106) were planted in mini-rhizotrons, in a standard potting compost mix. Five replicate trees were used per rootstock. Rhizotrons were used for two reasons; they allow roots to grow and develop without the restriction of a conventional plant pot. They also enable individual rootstock root systems to be sampled repeatedly for root hydraulic conductivity (khroot) measurements with minimal disturbance to the tree or the rest of the rootstock root system. The rhizotrons were placed in a randomized order on benching which kept the boxes at an incline of 20° from the vertical with removable access windows on the lower surface. All boxes were covered with reflective foil to reduce heat build-up and water was supplied automatically through a precision electronic controller and pressure compensating delivery values, i.e. ‘trickle irrigation’. The benching and rhizotrons were orientated north–south on concrete stands and maintained under ambient environmental conditions.
Measurements of root khroot were obtained from roots with diameters around 1.5 mm. From each of five replicate plants used, per rootstock, six roots of this size were removed for hydraulic analysis. The 1.5 mm diameter size class of root formed around 25% of the dry matter allocated to roots on a mass basis, but only 2–3% on a root length basis (data not shown). Roots of a diameter <1 mm formed more than 96% of the total root length, irrespective of rootstock type but were too small and delicate to measure khroot accurately.
In a second experiment 2-year-old grafted Malus trees were used. Clonal trees were produced by bud grafting all at the same height (15 cm) on the cultivar Queen Cox (self-fertile clone 18). Grafting was carried out in the field nursery, in situ, using 1-year-old rooted rootstocks. Three different types of rootstocks were used to encompass those that dwarfed scion extension shoots (M.27), those that were semi-dwarfing (M.9) and those that were semi-vigorous and promoted shoot growth (MM.106). After one year’s growth in the nursery, these grafted trees were planted in pots (10 dm3) and placed on sand/gravel beds and grown for a further year under ambient conditions at HRI-East Malling. During this period, watering was carried out automatically twice daily. The soil was kept moist by inspection and the length of irrigation events adjusted to avoid excessive run-off. After a further year, trees were repotted into larger volume pots. For the measurements of hydraulic conductivity, only trees in 15 dm3 pots were used, while the measurement of functional xylem area, using aqueous safranin solution, were carried out on trees, of the same age, in a range of pot sizes (i.e. 10, 15 and 25 dm3). Different pot sizes were used initially in order to examine the potential for different rooting volumes to influence conductivity. However, statistical analysis (ANOVA) for the different size pots showed that there were no statistically significant influences on functional xylem area due to pot size, only those due to rootstock, so all data were pooled. In total 15 replicate trees were used per treatment (rootstock), five per pot size.
Quantification of functional xylem area using safranin staining
After the termination of shoot extension, when the leaf canopy was at full expansion (mid August), trees were carefully removed from their pots with the root system intact and still within the compost. They were then sealed within a polythene bag and completely immersed in a large tank of water (300 dm3). Sealing the tree roots and soil within the bag minimized the contamination of the water with particulates, which could have occluded the xylem vessels when cutting the stem. The tree stems were cut under water at a point just above the first root, leaving as much ‘rootstock stem’ as possible (the ‘rootstock stem’ is tissue below the scion stem/root graft and is genetically root in origin). The cut rootstock stem was then placed into a beaker while still under water and transferred, without exposure to air, to a container of clean water. This was repeated to ensure no particle contamination had occurred. The tree was removed from the container with its stump within the beaker of water and clamped in a vertical position, in full sun, as it would have been when attached to its roots. Sufficient safranin dye in powder form was added to yield a 0.1% (w/v) aqueous solution (Sperry et al., 1988). The aerial parts of the tree were allowed to transpire freely for around 6 h. The transpiring trees were examined along with the solution reservoirs every 30 min to ensure the solution was being taken up and when necessary the aqueous safranin 0.1% solution was topped up with freshly prepared solution.
At the end of the experiment, similarly positioned radial sections of tissue were cut from each tree, within the rootstock stem, the middle of the graft tissue and within the scion stem (Queen Cox). This was repeated for each of the 15 replicate trees used per treatment. Stained scion stem sections were prepared (sanded) and the amount of tissue stained with safranin was quantified with an image analyser (Seescan Bioscience, Cambridge, UK). At low power magnification, entire radial stem sections were imaged and the area stained by the safranin determined relative to the total amount of xylem tissue. Fifteen trees were measured per rootstock.
Measurement of rootstock stem, graft tissue and scion stem hydraulic conductivity
Hydraulic conductivity (kh) was measured using the method described by Sperry et al. (1988). Roots, whole stem segments and subsequently stem samples were prepared under water (using the tank described above). Total leaf areas per tree were also determined after the stem sections had been taken. Initially, a stem section including rootstock stem, graft tissue and a section of scion was removed under water. The average sections lengths used were as follows; for rootstock shank, 50.4, 56.8 and 75.4 mm, the graft tissue, 53.0, 59.9 and 62.9 mm and scion stem, 58.8, 61.2 and 72.6 mm, for M.27, M.9 and MM.106, respectively. The average xylem vessel and fibre lengths for Malus are around 0.5 and 1.0 mm, respectively (Loach, 1960). This would appear to be on the short side, relative to other diffuse porous species, where the largest proportion of vessels are between 0–10 cm (Zimmermann and Jeje, 1981). Total conductivity of this complete section (khrgs) was measured first, the section was re-cut under water to remove a known sample length of rootstock stem tissue and khr was measured. Finally, a sample of scion stem tissue from the original section was removed under water and its hydraulics measured (khs). The cut ends of all stem sections were cleaned and prepared with a razor blade prior to transfer to the measurement tank.
The measurements made from these stem sections were used to determine the hydraulic conductivity of the graft tissue, which could not be measured directly due to its size and shape. This was achieved by assuming that the rootstock stem, the graft tissue and the scion stem sections could be considered as a series of three resistances, as follows: Rrgs=rootstock stem+graft tissue+scion stem resistance; Rr=rootstock stem resistance; Rs=scion stem resistance; Rg=graft tissue resistance (unknown).
The resistance of the segments was determined from measured kh values knowing that Resistance=1/conductance and conductance =kh/tissue length.
The unknown graft tissue resistance (Rg) was determined as follows:
Rrgs=Rr+Rg+Rs(1)
and thus
Rg=Rrgs–(Rr+Rs)(2)
substituting into equation (2) Resistance=tissue length/kh
which rearranges to:
Hydraulic conductivity was measured in a gravity-fed flow system, in which the pressure across the stem section could be varied (the measurements were made around 6 kPa). Degassed and filtered (0.2 µm mesh Millipore filter) oxalic acid solution (10 mol m–3) was used as the perfusion solution to prevent any long-term decline, due to microbial growth, in conductivity (Sperry et al., 1988). The mass of solution flowing per unit time through the stem segment was recorded continuously with an electronic balance, to five decimal places. This value, and the pressure head, the length of the segment (mm) and the temperatures (approximately 20 °C) were all used to calculate continuous measurements of kh (flow rate through a given length of sample per unit pressure gradient), i.e.
Measurements of mass flow were made every 10 s for each sample until the coefficient of variation had declined to <2%. At this point the final three readings were averaged to estimate the hydraulic conductivity (kh; kg m s–1 MPa–1). Hydraulic calculations were made using a programme written by Tyree and modified by Cochard with manual data inputs of temperature and reservoir head height.
Stem hydraulic conductivity was subsequently used to calculate stem specific conductivity (kss) by dividing khs by xylem cross-sectional area (kss; kg m–1 s–1 MPa–1). Leaf specific conductivity (ksl) was also determined by dividing khs by the ‘supported leaf area’; ksl is a measure of xylem hydraulic supply capacity (Zimmermann, 1983). This approach follows that used by Tyree and Ewers (1991). Eight trees were analysed per rootstock.
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Hydraulic conductivity of excised individual roots from ungrafted rootstocks with different size controlling capacities
The hydraulic characteristics of roots from a dwarfing and a semi-vigorous rootstock when grown in rhizotrons are shown in Table 1. Comparing roots of the same size class but from different rootstocks, showed that roots from dwarfing rootstocks had significantly lower khroot (by about 50%) and these differences remained when the results were expressed as ksroot, i.e. relative to root cross-sectional area (Table 1). These measurements however, only account for a small part of the root system and may not reflect the properties of the entire root system. Stem sections from the same trees also showed differences in khs and kss values for material from the more vigorous rootstocks compared to that from the dwarfing rootstocks (Table 2). Due to the high variability in the MM.106 measurements, the rootstock effect was not significant at the 0.05% level. khs was also expressed relative to total plant leaf area (ksl) and total root length per plant (ksr); differences between the two rootstocks were small and were not statistically significant.
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Quantification of functional xylem area using safranin staining
Xylem staining with aqueous safranin combined with quantitative image analysis enabled rapid measurements of the amount of functional xylem tissue. Observations from stem sections showed that the rootstock stem, irrespective of vigour, had a high proportion (>60%) of the xylem area stained with dye (Table 3). The exception was the central xylem core of the rootstock stem, which was non-functional. This tissue corresponds to the original xylem present (first years growth prior to scion grafting) when the main stem of the rootstock was cut off after the graft established. Sections taken in the middle of the graft tissue and in the scion stem showed that the ratio of total xylem area to stained tissue declined with increasing vigour of the root type, i.e. the proportion of stained tissue increased (Table 3). The total area of stained stem xylem, calculated as a percentage, was significantly greater for the semi-vigorous rootstock (MM.106, 47%) compared to the dwarfing (M.27, 24%). The ratio of stem diameter between the graft tissue and the rootstock stem and scion stem showed differences in diametric growth size (Table 3). In all cases, however, even with these comparatively juvenile trees (3-year-old) the graft tissue had the greater diameter compared to the scion stem or rootstock stems.
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Quantification of rootstock stem, graft tissue and scion stem hydraulic conductivity
Scion stem khs increased with the vigour of the rootstock on which it was grafted. Mean khs values, from at least eight trees within each of three rootstock types, were 5.63±0.28, 7.57±0.45 and 15.24±0.94 kg m s–1 MPa–1x10–4 for M.27, M.9 and MM.106, respectively. As the total tree leaf area of these grafted trees increased significantly in relation to the rootstock’s vigour, khs was also expressed in relation to the ‘supported leaf area’, i.e. the total leaf area above the stem section measured (Fig. 1A). The highest khs values were evident with stems supporting the largest leaf areas and the most vigorous rootstocks, i.e. MM.106, while the lowest khs values measured were from stems on dwarfing rootstocks (M.27), which had the lowest leaf area. kh were also calculated per unit xylem cross-sectional area (stem specific conductivity kss) to remove the potential influence of differences in radial diameter of the xylem tissue sections sampled (Fig. 1B). There were, in 3-year-old trees, only relatively small differences in stem diameter associated with rootstock vigour. The relationship between stem kss and leaf area was still positive with kss being higher in scion stems grafted onto semi-vigorous (MM.106) compared to dwarfing rootstock (M.27).
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Stem hydraulics were also calculated independently of stem section diameter, and expressed as leaf specific hydraulic conductivities (ksl) and are shown in Table 4. ksl was nearly twice that for scion stem sections grafted on semi-vigorous (MM.106) compared to that on the dwarfing rootstock (M.27). ksl values, calculated for the semi-vigorous and dwarfing entire stem sections (rootstock stem, graft union, scion stem) and rootstock stem showed an 8–10-fold increase for the semi-vigorous rootstock.
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The efficiency of xylem tissue in supporting a transpiring leaf surface was determined by relating leaf area to xylem cross-sectional area. The ability of a unit area of xylem to supply an amount of leaf area was determined using xylem cross-sectional areas for the rootstock stem, the graft tissue and the scion stem (Table 5). The efficiency with which leaf area was supported by rootstock stem was very similar (around 35 cm2 of leaf area mm–2 of xylem CSA), irrespective of rootstock type. When considering the efficiency of the graft tissue, 1 mm2 of xylem of dwarfing M.27 rootstock supported significantly less leaf, i.e. around 60% of the semi-vigorous MM.106 rootstock. The semi-dwarfing M.9 rootstock had a slightly higher value intermediate between the dwarfing and semi-vigorous rootstock. A unit area of scion xylem, of the semi-dwarfing and semi-vigorous rootstocks, supported significantly more leaf area (around 20%) than that evident for the dwarfing rootstock (Table 5).
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Measurements of hydraulic conductivity across the entire grafted stem and rootstock sections (khrgs), which included the rootstock stem, the graft tissue and a section of the scion stem, showed that mean khs increased with the vigour of rootstock used in the grafting (Fig. 2A). These entire stem sections from trees grafted onto dwarfing rootstocks (M.27) had khrgs values of 1 kg m s–1 MPa–1 x 10–4 compared to 14.6 kg m s–1 MPa–1x10–4 for trees on semi-vigorous rootstocks (MM.106), while those on the semi-dwarfing (M.9) were intermediate at 2.7 kg m s–1 MPa–1x10–4. Measurements of a subsequently detached rootstock stem section, showed that khr values increased in relation to rootstock vigour and in similar magnitude, as evident with scion stem section conductivity (khs) (see below, Fig. 2A). These differences were still evident when differences in total leaf area were taken into account (data not shown).
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The kss values were reciprocated to express conductivity as resistivity and are shown in (Fig. 2B). These values were much greater for all sections taken from the dwarfing rootstock (M.27) compared to the semi-vigorous rootstock (MM.106). Using xylem specific conductivities (ks) enables rootstock anatomical properties to be determined independently of differences in stem cross-sectional area. The resistivity of the dwarfing rootstock, measured over the complete scion stem section (r+g+s) was of the order of 20 times greater than for the semi-vigorous rootstock. When kh was used to determine the actual conductivity of the graft tissue, as described in the materials and methods section (derived from equation 4), the dwarfing rootstock (M.27) had a mean calculated conductivity of 2.18 kg m s–1 MPa–1x10–4 (Table 6). This was less than half (4.36 kg m s–1 MPa–1x10–4) that of the semi-dwarfing rootstock (M.9). The graft tissue of the semi-vigorous rootstock, however, (MM.106) had a conductivity that was 4 and 9 times greater, respectively, than the semi-dwarfing and dwarfing rootstocks. These calculations do not take into account the differences in xylem cross-sectional area as reported in Table 3, but enable conductivity comparisons to be made of the different component parts of the grafted tree.
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The hydraulic conductivity of roots from dwarfing rootstocks was lower than those measured from invigorating rootstocks. This confirms what has been implied from early anatomical observations of roots (Beakbane and Thompson, 1947). This study’s measurements of khroot values were similar to those quoted for excised roots, for a range of species (Rieger and Litvin, 1999). Even when expressed relative to differences in root cross-sectional area, ksroot values were still lower in roots from the dwarfing compared to the semi-vigorous rootstocks. This is similar for root kh of citrus rootstocks which decreases with the capacity to dwarf grafted stems (Syvertsen and Graham, 1985). These observations are consistent with the view that clonally produced dwarfing rootstocks possess innate factors, such as lower xylem to phloem ratios and changes in xylem vessel anatomy (Beakbane and Thompson, 1947), which might explain how they influence shoot behaviour when used in grafted plants. However, some caution should be used because these measurements are a reflection of a size class of root, which reflects only a relatively small part (25%) of the entire root system, at least with respect to measurements of root length. Ninety-six per cent of the remaining root, determined by length, was <1 mm in diameter and would probably have had a greater influence on total root system hydraulics. Recent data suggests that entire dwarfing root systems of Malus, have lower hydraulic conductivities than more vigorous ones but such differences are lost when root mass is accounted for (MA Else et al., unpublished data). It should also be noted that for the expression of root hydraulic conductivity relative to cross-sectional area, it has been assumed that the xylem to total root cross-sectional area was the same for each rootstock, evidence suggests they may not be (Beakbane and Thompson, 1947).
The velocity of basipetal auxin transport is lower in the dwarfing M.9 rootstock compared to the more vigorous MM.111 (Soumelidou et al., 1994b). A trend for the concentration of IAA in bark tissue to decline in more dwarfing apple rootstocks has also been found (Kamboj et al., 1999). IAA concentrations were also lower in shoot tips of dwarf Malus mutants (Jindal et al., 1974). The physiological differences in kh measured for rootstock roots agrees with the earlier growth studies of Tubbs (1973a, b) where it was argued that the rootstock influence on scion stem growth was independent of the scion and greater than that of the scion on rootstock growth.
Quantification of the area of functional xylem in stem sections by staining with aqueous safranin solution proved to be very informative. The staining and apparent movement of safranin solution in the rootstock stem sections was confined to functionally active xylem. This suggests that within the rootstock stem there was little if any radial xylem flow under these experimental conditions. More importantly, however, was the measured reduction in stained area of the scion stems grafted on to dwarfing rootstocks (M.27) relative to those on semi-vigorous rootstocks, indicating a reduction in the functional area of xylem above the graft union. This effect could explain why there is a decline in kh across the graft tissue.
Quantitative measurements of stem kh were similar to those obtained for Acer saccharum trees with comparable stem diameters (Yang and Tyree, 1994). It has also been shown that the vigour of the rootstock directly influenced the stem’s hydraulic conductivity. This difference in kss occurred in stems independently of changes in tissue stem diameter or the influence of differences in leaf area. Calculated values of ksl were greater for the semi-vigorous rootstock (MM.106) compared to the dwarfing rootstock (M.27), reflecting a lower pressure gradient (dP/dx, MPa m–1) supplying water to the leaves of the former. The calculation of Huber-type (the amount of leaf area supported by area of stem tissue) values also indicated that a unit of scion xylem, on a semi-vigorous rootstock, was able to support a greater leaf area than that on a dwarfing rootstock (M.27). This difference in leaf area supported was evident when expressed relative to graft tissue or scion stem tissue size (CSA), but not rootstock stem.
Measurements of stem hydraulics across an entire stem section of a combined resistance series, from the rootstock stem, through the graft tissue, to the scion stem, showed that conductivity was related to rootstock vigour. This agrees with studies quantifying the movement of aqueous safranin solution across the graft tissue and those suggested by Warne and Raby (1938). Trees with semi-vigorous rootstocks had the highest conductivity. When the graft tissue conductivities were calculated, by difference, the largest factor contributing to the variation in kh between rootstocks for the grafted stems was the graft tissue itself.
The differences in kh measured here support suggestions made from observations of rootstock graft tissue anatomy, where abnormal xylogenesis was very apparent with M.9, but not with MM.106 (Soumelidou et al., 1994a). These data also show that the hydraulic conductivity of graft tissue from a semi-vigorous rootstock (MM.106) was much greater than that of a dwarfing rootstock (M.27). The hydraulic conductivity of the semi-dwarfing rootstock (M.9) was intermediate between the other rootstocks.
It is likely that differences in intact stem conductivity and, therefore, sap flow in the xylem between rootstocks, may be less evident as the tree ages. In such cases the graft tissue cross-sectional area frequently increases with dwarfing rootstocks. An increase in the diametric growth of graft tissue may be a mechanism by which the scion stem (or more likely the amount of transpiring leaf area) on a dwarfing rootstock could overcome the hydraulic limitations imposed by the graft tissue and its abnormal xylem anatomy.
This research has shown that the hydraulic conductivity of the measurable roots from a rootstock may vary slightly, but it is difficult to determine the impact of this on the entire root system. Despite this, rootstocks have been shown capable of influencing scion hydraulics independently of differences in leaf area. Observations using staining to determine the amount of functional xylem, show that there was little innate difference with respect to rootstock vigour. But the amount of functional xylem tissue in the graft and scion increased with rootstock vigour. This was reflected in the measurements of scion stem hydraulics even when differences in xylem cross-sectional area were taken into account. This suggests that such changes were due to anatomical features associated with xylem anatomy and not simply due to differences in stem cross-sectional area.
These observations show that graft tissue of a dwarfing rootstock has a lower hydraulic conductivity compared to a more vigorous rootstock. In order to overcome the anatomical disfunction associated with the xylem anatomy of graft tissue, the cross-sectional area of the union increases. This is a typical auxin response, which is probably due to an imbalance caused by greater basipetal transport of auxin in the scion, relative to the transport in dwarfing rootstock (Soumelidou et al., 1994b). This leads to the accumulation of auxin in the graft tissue, which explains the increased graft xylogenesis.
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Acknowledgements |
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This work was funded by the Ministry of Agriculture, Fisheries and Food. We are extremely grateful for the comments of Drs Mike Fordham, Tony Webster and David Dunstan on an earlier draft of this manuscript and Gail Kingswell for the statistical analysis.
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